Molecular sorting in polarized and non-polarized cells: common problems, common solutions.

We have been interested in determining how epithelial cells generate and maintain their characteristically polarized distributions of membrane proteins. Our efforts to date strongly indicate that the polarized transport in MDCK cells may be due to a set of discrete targeting determinants often found on a membrane protein's cytoplasmic domain. Surprisingly, these determinants are widely distributed and are not specific to proteins expressed in polarized cells. They also appear to function in controlling polarized transport along both the biosynthetic and the endocytic (or transcytotic) pathways. Signals for basolateral transport have been characterized and, like the cytoplasmic domain signal used by plasma membrane receptors for accumulation at clathrin-coated pits, they often involve a critical tyrosine residue. Although the basolateral and coated pit signals may also be co-linear, they are not identical. The basolateral and apical transport determinants are also hierarchically arranged. Although a single protein may contain one or more signals specifying basolateral transport, inactivation of these signals appears to reveal a determinant that directs efficient apical transport. Given that the sequence determinants responsible for polarized transport are not restricted to epithelial cells and are related to determinants commonly utilized in all cells, it is possible that non-polarized cells contain cognate apical and basolateral pathways that are responsible for 'constitutive' transport from the Golgi to the plasma membrane. The presence of two cognate pathways might confer a high degree of plasticity to pre-differentiated cells, allowing them rapidly to begin assuming a polarized phenotype in response to extracellular stimuli without requiring the synthesis of epithelial cell-specific transport machinery.